Had the same over here and found a solution. You probably have a non US Windows version and the installer is missing the appropriate RevitInstallXXX.dll where the XXX stands for the language code.
1. download webinstaller but don't run it
2. instead unzip the exe and put in a directory.
3. copy your RevitInstallXXX.dll file from where you have installed 5.1 to this directory
4. start installation with RevitInstall.exe